Mammals, birds, and several species of bacteria incorporate selenium as selenocysteine sites of a few essential proteins. The bacterium Escherichia coli produces a selenocysteine-containing enzyme, formate dehydrogenase, when grown under anaerobic conditions. We have used this as an easily manipulable model system for analyzing the regulation of gene expression at the transcriptional and translational levels. In contrast to the results found for other anaerobic-specific genes, we have found that inhibition of gyrase activity (which increases the supercoiling of DNA) enhances the expression of formate dehydrogenase. We have purified this formate dehydrogenase to near homogeneity. This has allowed studies on the properties of this selenoprotein. The chemical function of the selenocysteine moiety in the enzyme's reaction mechanism can be analyzed by comparison of this protein with a mutant protein in which the selenocysteine is replaced by cysteine. This mutant species has been purified recently. We have found this mutant enzyme binds its substrate with an affinity similar to the wild-type enzyme, but its catalytic activity is greatly reduced. Elucidation of the biochemical mechanism of selenium utilization would allow a greater appreciation of the essential role of selenium in the diet. With an understanding of the mechanism of selenocysteine incorporation into protein, one could direct the mutagenesis of a protein such that selenocysteine replaces cysteine. Such protein engineering could significantly alter the catalytic properties of many enzymes.